lo A MANUAL OF ANATOMY 



at the margin of which it Ceases. It consists of two layers — outer 

 and inner. The fibres of the outer layer are arranged closely, and 

 it is therefore dense. The inner layer consists chiefly of ramifying 

 elastic fibres. Between the inner layer and the surface of the 

 shaft of a long bone there is, especially during the development 

 and growth of the bone, a layer of subperiosteal areolar tissue. 

 Within its meshes cells, called osteoblasts, accumulate during the 

 period of growth. These cells emerge from the outermost Haversian 

 canals along the course of the entering arteries, and they represent 

 the hotie cells of the bone-lacunae of compact bone which have 

 migrated outwards. The osteoblasts take an important part in 

 the formation of bone during its growth. 



The periosteum is richly supplied with arteries which, after ramify- 

 ing, enter the outermost Haversian canals, along with fine processes 

 of the inner periosteal layer. The periosteum serves (i) as a bed 

 in which the arteries subdivide before entering the bone, and (2) 

 to give a firm hold to tendons and ligaments. It also takes part 

 in ossification and regeneration of bone. When the periosteum is 

 stripped from a bone the uncovered portion is liable to necrosis 

 and exfoliation. 



Marrow or Medulla. — The marrow fills the marrow canals of long 

 bones and the medullary spaces of cancellated bone, and it also 

 sends processes into the innermost Haversian canals. It is composed 

 of a reticular fibrous matrix, which is pervaded by many blood- 

 vessels and cells, the latter being called myelocytes (marrow cells). 

 There are two kinds of marrow, yellow and red, which differ as to 

 the character of the cells. In yellow marrow most of the cells have 

 become transformed into fat ceUs, so that the marrow resembles 

 adipose tissue. Red marrow contains very few fat cells. Many of 

 its cells are colourless, protoplasmic, nucleated cells, which resemble 

 the leucocytes of the blood, though of larger size, and like them are 

 capable of amoeboid movement. Other reddish cells, called ery- 

 throblasts, are present, which are the sources from which large 

 numbers of red blood corpuscles are formed. In addition to these 

 two sets of cells, there are large multinucleated, protoplasmic cells, 

 called the myeloplaxes of Robin, which play an important part in 

 the absorption of bone. 



Yellow marrow is found in the marrow canals of long 

 bones, whilst red marrow occurs in (i) articular ends of long 

 bones ; (2) medullary spaces of cancellated bone ; (3) bodies of 

 the vertebrae; (4) sternum; (5) ribs; and (6) the diploe of the cranial 

 bones. Marrow serves the following uses: (i) it (red marrow) is 

 an important blood-forming organ (red corpuscles); (2) it contributes 

 to the nourishment of bone; and (3) it serves as a light packing 

 material for all hollow spaces within bones, with the exception of 

 the air-sinuses in the bones of the head. 



The wall of the marrow canal of all long bones and that 

 of the medullary spaces of all cancellated bone are lined with 

 a very delicate layer of areolar tissue, which is richly provided 



